1. Field of the Invention
This invention relates to intraocular lenses. The invention also relates to methods of making such lenses, and to copolymers useful, for example, in intraocular lenses.
2. Description of Related Art
Various types of intraocular lenses (IOLs) are known. For example, there are known one-piece IOLs and composite IOL having multiple pieces. A one-piece IOL is one where both optic and non-optic portions are made from one material. The non-optic portions are referred to as haptic portions, and are used for attachment purposes. Two general designs for the haptics are a "plate-type" and a "C-haptic" type, both of which have a variety of shapes. A plate design is shown in FIGS. 1A and 1B. A "C" design is shown in FIGS. 2A and 2B. These Figures are discussed in more detail below.
It is desired to reduce the length of incision needed to insert the IOL into the eye. Reducing the length of the incision requires the use of a very soft material for the lens; soft enough to permit the lens to be folded, inserted through the incision, and released. Thus, intraocular lenses are preferably foldable so that they can be easily inserted into the eye. Approaches to achieve foldable materials include use of lenses formed of hydrophobic silicone-polymers, certain hydrophilic 2-HEMA homopolymers, and composites of a hydrophilic optical core with hydrophobic poly(methyl)methacrylate (p-MMA) based haptics.
IOLs with mechanically-attached haptics are lenses where the optic-portion and the haptic-portion are made separately, usually from different materials, and attached. For example, the lens portion can be made of a hydrogel or silicone-based material and the C-shape haptics from a rigid material like p-MMA. The p-MMA haptics are attached to holes drilled into the optic portion.
Although P-MMA is the traditional hydrophobic IOL it is not easily foldable and requires a relatively large incision for insertion. To make hydrophobic materials foldable, there has been included rubber-like materials, such as silicone derivatives, into the rigid polymer matrix or use of materials consisting primarily of silicone derivatives. Although the softness of a primarily silicone material is ideal for folding prior to insertion, when the lens and its haptics unfold in the eye, the almost gel-like softness of the lens makes it difficult for a surgeon to properly position the lens in the eye. Furthermore, the silicone lens often does not provide sufficient rigidity for the lens after insertion and the combination of deformation from compressive forces along with lens movement can produce lens distortion and compromise the optical integrity of the lens.
Because of its inherent properties, p-hydroxyethyl methacrylate (a homopolymer of HEMA) has been used as a foldable material for IOLs. However, the low refractive index of p-HEMA when hydrated leads to limitations in the optical center design and a compromise between its folding ability and its optical requirements.
One of the limitations of one-piece P-HEMA hydrogel lenses has been that the haptic portion lacks the desired dimensional stability and can compromise lens positioning. To address this issue, polymer materials have been combined to give a soft, foldable intraocular composite lens such as P-HEMA or other soft acrylic material for the optic zone, and a rigid structure around the outside of the lens, made from a hard plastic such as P-MMA. See U.S. Pat. No. 4,718,906 and U.S. Pat. No. 5,326,506, both hereby incorporated by reference in their entireties, which describe composite IOLs. These multicomponent materials are made by embedding one material in the other, or by concurrent extrusion processes, or by solidifying the hard material about the soft material, or by forming an interpenetrating network of the rigid component into a preformed hydrophilic core.
U.S. Pat. No. 5,326,506, discloses a composite intraocular lens including a soft, pliable inner lens optic using rigid haptics. The lens optic material is a low water content material such as a copolymer of 2-HEMA and hydroxy hexylmethacrylate which has a high refractive index due to its low water content. The hard yet foldable P-MMA haptics, are formed by an interpenetrating network.
An article by Chirila et al., J. Cataract Refract. Surf., pp. 154-162, Vol. 17, March 1991 discusses the toxicity effects of residual monomers, such as 2-hydroxyethyl methacrylate, methyl methacrylate, and 2-ethoxyethyl methacrylate, in IOLs. One piece IOLs are not proposed.
U.S. Pat. No. 4,764,169, hereby incorporated by reference in its entirety, discloses a composite intraocular lens including a small, hard inner lens optic and a soft, pliable skirt surrounding the lens optic. The lens optic material is a relatively hard material such as P-MMA, polysulfone, or polycarbonate. The soft, pliable skirt is a silicone, hydrogel or like material.